The production of heavy oil and bitumen from a subsurface reservoirs such as oil sands or shale oil is challenging. One of the main reasons for the difficulty is the viscosity of the heavy oil or bitumen in the reservoir. At reservoir temperature the initial viscosity of the oil is often greater than one million centipoises, which is difficult to produce if not mobilized using external heat. Therefore, the removal of oil from the reservoir is typically achieved by introducing sufficient energy into the reservoir to heat the reservoir, such that the viscosity of the oil is reduced sufficiently to facilitate oil production.
Currently the preferred method of introducing energy into the reservoir is steam injection. The heat from the steam reduces the viscosity of the fluid, allowing it to flow toward production wells. The steam also provides voidage replacement to maintain the pressure in the reservoir. There are several variations on steam injection methods of producing heavy oil, including Cyclic Steam Stimulation (CSS), steam drive, and Steam Assisted Gravity Drainage (SAGD), but all of these methods use steam for heating and maintaining pressure in the reservoir.
Cyclic steam stimulation or CSS was accidentally discovered by Shell while doing a steam flood in Venezuela and one of its steam injectors blew out. The well produced oil at much higher rates than a conventional production well in a similar environment, leading to the realization that steam injection could improve production. CSS, also known as the Huff and Puff method, consists of 3 stages: injection, soaking, and production. Steam is first injected into a well for a certain amount of time to heat the oil in the surrounding reservoir to a temperature at which it flows. After it is decided enough steam has been injected, the steam is usually left to “soak” for some time after (typically not more than a few days). Then oil is produced out of the same well, at first by natural flow (since the steam injection will have increased the reservoir pressure) and then by artificial lift. Production will decrease as the oil cools down, and once production reaches an economically determined level the steps are repeated again. This process is shown schematically in FIG. 1.
CSS can be quite effective, especially in the first few cycles. However, it is typically only able to recover approximately 20% of the Original Oil in Place (OOIP), compared to steam flooding, which has been reported to recover over 50% of OOIP. It is quite common for wells to be produced in the cyclic steam manner for a few cycles before being put on a steam flooding regime with other wells.
CSS and steam flooding are quite distinct processes in the petroleum industry. In a steam flood, sometimes known as a steam drive, some wells are used as steam injection wells and other wells are used for oil production. Two mechanisms are at work to improve the amount of oil recovered. The first is to heat the oil to higher temperatures and to thereby decrease its viscosity so that it more easily flows through the formation toward the producing wells. A second mechanism is the physical displacement employing in a manner similar to water flooding, in which oil is meant to be pushed to the production wells. While more steam is needed for this method than for the cyclic method, it is typically more effective at recovering a larger portion of the oil.
Radio frequencies (RF) have been used in various industries for a number of years. One common use of this type of energy is the household cooking appliance known as the microwave (MW) oven. Microwave radiation couples with, or is absorbed by, non-symmetrical molecules or those that possess a dipole moment, such as water. In cooking applications, the microwaves of about 2.4 GHz are absorbed by water present in food. Water vapor molecules, in contrast, are known to absorb at about 22 GHz. Once the water absorbs the MW, the water molecules rotate and generate heat, thus heating the remaining molecules through a conductive heating process.
RF has also been used in various downhole applications, but to our knowledge has never been applied in a CSS method to improve CSS efficiencies.
U.S. Pat. No. 6,189,611 describes the application of cyclically applied RF energy radiated at a power of 10 kilowatts (KW) and a frequency of 27.12 megahertz (MHz). When the temperature at the applicator well reaches about 140° C., the radiation power is cycled down to 8 to 9 KW, typically for a period of several hours, until the temperature of the applicator well cooled to about 130° C., and then the power was cycled back to 10 KW. The inventors describe the production of oil as occurring in spikes, similar to the way oil is produced in huff and puff methods. However, this method is not a true combination of CSS and RF reheating. Instead, it uses RF to replace steam injection. Thus, the method fails realize the benefits of combining CSS with RF reheating.
Thus, there exists a need to combine the technology of conventional cyclic steam stimulation with RF technologies to both increase the amount of oil produced from the reservoir and in situ upgrade the oil in the reservoir, while reducing the time required for draining the water condensed from the injected steam and re-injection of steam for next cycle.